UNIVERSITY OF GOTHENBURG Department of Earth Sciences Geovetarcentrum/Earth Science Centre Validation of two Numerical Ocean Models in Skagerrak Lisa Svenningsson ISSN 1400-3821 B712 Master of Science (Two Years) thesis Göteborg 2012 Mailing address Address Telephone Telefax Geovetarcentrum Geovetarcentrum Geovetarcentrum 031-786 19 56 031-786 19 86 Göteborg University S 405 30 Göteborg Guldhedsgatan 5A S-405 30 Göteborg SWEDEN Abstract A new high-resolution numerical ocean model over the north eastern Skagerrak region, with focus on the Koster fjord national park area, is in need of lateral boundary conditions. The suitability of two different 30-year long model simulations have been examined, namely (i) the BaltiX model, a NEMO based configuration developed by SMHI (Swedish Meteorological and Hydrological Institute, Norrköping, Sweden) and (ii) a MIKE3 model by DHI (Danish Hydrological Institution). A comparison of these two models to observational data is made to see which one of these models is more suitable as outer boundary condition for the high-resolution model. Since the salinity field is known to be of great importance for the dynamics in Skagerrak, this study has been focused on the vertical salinity distribution. The conclusions are that both models have lower salinity than observations but apart from that seem to capture many characteristics of the observed stratification. The surface variability of both models also seems reasonable in comparison to statistics of observations. The BaltiX model suffers from low salinity in the entire water column in the beginning of the year and seems to have some problem transporting the low salinity surface water out from Skagerrak during the later months of the year. MIKE3 is the stronger candidate of the two in most comparisons but still has extensive problems with low salinity. It is also shown that both models simulate less concentrated costal currents than the observed. 1 Table of Contents 1. Introduction ......................................................................................................................................... 3 1.1 Study area ...................................................................................................................................... 3 1.1.1 Hydrography ........................................................................................................................... 4 2. Models and Validation Methods ......................................................................................................... 4 2.1 BaltiX .............................................................................................................................................. 4 2.2 MIKE3 by DHI ................................................................................................................................. 6 2.3 Validation method ......................................................................................................................... 6 2.3.1 Salinity profiles ....................................................................................................................... 8 2.3.2 Freshwater height .................................................................................................................. 8 2.3.3 Single depth qualitative comparison ...................................................................................... 8 2.2.4 Short-term variability comparison ......................................................................................... 8 3. Results ................................................................................................................................................. 9 3.1 Salinity profiles .............................................................................................................................. 9 3.2 Freshwater height ......................................................................................................................... 9 3.3 Single depth qualitative comparison ........................................................................................... 10 3.4 Comparison of short-term variability between the models ........................................................ 12 4. Discussion .......................................................................................................................................... 13 5. Conclusions ........................................................................................................................................ 14 6. Acknowledgement ............................................................................................................................. 14 7. References ......................................................................................................................................... 15 Appendix I .............................................................................................................................................. 16 Appendix II ............................................................................................................................................. 21 2 1. Introduction The Skagerrak and the North Sea region is a well investigated area and has for a long time been the subject of many marine research projects in the surrounding countries. Several numerical oceanographic model studies have been carried out over the North Sea area and the Skagerrak region throughout the years. A new high-resolved three-dimensional model study is now planned for the north eastern Skagerrak region with focus on the Koster fjord national park area. The purpose is to study the spreading of free floating particles such as cod fish eggs and planktonic larvae. Since mesoscale structures is known to influence the dispersion pattern of free floating substances, a resolution high enough to resolve eddy structures is important (Albretsen and Røed 2010). Several comprehensive validation projects of model simulations have been carried out in the Skagerrak area over the years. Some examples of such are the Skagex project 1995 (Svendsen et al. 1995), a validation experiment by Gothenburg university in collaboration with the Swedish environmental protection agency, the Norwegian metrological institute and Danish hydraulic institute (Gustafsson et al. 2000) and a comprehensive investigation concerning simulations of mesoscale structures by Albretsen and Røed, 2010. The most interesting of these, regarding particle dispersion, is the latter since it focuses on finding a resolution sufficient to capture mesoscale structures, such as eddies, current jets and filaments to simulate pathways and the statistical properties of the observed patterns. A numerical oceanographic model, like the one planned for the Koster fjord national park area, is in need of forcing data on the open boundaries. For this, two 30-year long model simulations with a computational domain over the North and the Baltic Sea are available. The BaltiX model, a NEMO based configuration developed by SMHI (Swedish Meteorological and Hydrological Institute, Norrköping, Sweden) and a MIKE3 model by DHI (Danish hydrological institution). The computational domain as well as the area of the high-resolved model is shown in figure 1. To investigate which one of these models is best suitable to use as forcing in the highly-resolved model, they need to be evaluated in comparison to observations. The aim of this thesis is to compile existing knowledge and data of the hydrographic conditions in the Skagerrak area and to examine model runs from BaltiX and MIKE3 which are candidates for being used as outer boundary forcing datasets for the high-resolved study. 1.1 Study area The Skagerrak is located between the Danish Jutland, the Swedish west coast and southern Norway. In the south it borders to Kattegat, which in turn is connected to the Baltic Sea through the Danish straits. In the west, Skagerrak has a border to the North Sea (see figure 1). The basin is significantly deeper than the surrounding areas with a maximum depth of approximately 700m and an average depth of 210m (Rodhe, 1996). The most prominent topographical feature of the region is the Norwegian trench outside the southern tip of Norway. A more detailed description of the topographical features of Skagerrak is given by Rodhe (1996). 3 1.1.1 Hydrography The main characteristic feature of the hydrography of Skagerrak is a strong salinity stratification in the upper layer. The fresh surface layer, 10-100m deep, with salinity <35 psu, is a result of three freshwater sources. The most important of them is the north bound Baltic current of low saline water flowing northwards along the Swedish cost carrying about 15 000 m3s-1 of freshwater from the Baltic Sea (Svansson, 1975). The other two freshwater sources are river run-off from local areas, corresponding to 2 500 m3s-1, and low salinity surface water from the southern North Sea that circles the basin (Rydberg et al. 1996). The low salinity in the southern North Sea is due to continental river discharge. The circulation of the upper freshwater influenced layer has a mean cyclonic movement, as does the deep water. The volume of the low-salinity flow from Kattegat has been shown to increase about eight times before leaving the Skagerrak along the Norwegian coast. This illustrates how important the diapycnal mixing mechanism is for the characteristics of the freshwater influenced layer in the Skagerrak (Gustafsson and Stigebrant, 1996). The